The present invention relates to techniques for performing wellbore operations. More particularly, the present invention relates to techniques for determining characteristics of subterranean formations using acoustic wellbore data.
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present disclosure, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions.
In a well completion, a string of casing or pipe is typically set in a wellbore, and an annulus fill material (e.g., cement) is disposed in the annular space between the casing and the earth formation. One objective of filling the annular space is to separate oil- and gas-producing layers from each other, and from water-bearing formation strata. If the cement fails to provide isolation of one zone from another, fluids under pressure may migrate from one zone to another, reducing production efficiency. Also, migration of hydrocarbons into aquifers is environmentally and economically undesirable. Evaluating the annulus content is important for reliable determination of the zonal isolation of the different strata of a formation.
Acoustic evaluation is often used to determine whether the cement provides hydraulic zonal isolation between formation strata traversed by the drilled well. Certain acoustic measurements, such as the ultrasonic pulse-echo measurement, have been widely used in cement evaluation to provide the effective acoustic impedance of the annulus material adjacent to the casing with high azimuthal and axial resolution and thereby evaluate cement characteristics and zonal isolation. For example, an ultrasonic pulse-echo tool may transmit a broadband pulse, usually between 200 and 700 kHz, to the casing wall to excite a thickness resonance mode in the casing. The acquired signals may be processed using modeling techniques to estimate the annular acoustic impedance. However, some conventional approaches are more suitable for thinner casings (e.g., casings thinner than 12 mm), and have not been as suitable for the larger and thicker casings in acoustically-attenuative (heavier) borehole muds that are more commonly used today. Such wellbore environments can result in acoustic behavior that reduces the accuracy of cement evaluation.